Historically, the techniques of traditional glassblowing have remained a safely guarded secret. "Over centuries they were passed down from father to son behind closed doors," says Peter Layton founder of the London Glassblowing Workshop. "In medieval times, Italian glassmakers were contained on the island of Murano. There are stories of death squads chasing errant glassmakers to Holland."

Thankfully, this isn't still the case. Instead, here they positively encourage people to learn the traditional techniques, and run classes for the public.

The workshop, in the old Leather Market in south-east London, is large, bright and very, very hot. The furnace – where white, sandy pellets are melted to form clear glass – runs at 1,200C. The heat is almost unbearable.

Layton dips what is known as the "punty" iron – a long iron rod – into the furnace and "gathers" some molten glass on the end. As he pulls the punty back out he rotates it continuously to prevent molten glass from falling to one side. With his other hand he uses what looks like an enormous set of iron tweezers (known as a jack) to draw out strands of glass. It's how fibreglass is made, he explains and then drops this demonstration sample into a bin where it later explodes and threatens to catapult across the room. Molten glass shatters unless it is cooled gradually overnight in the "annealing oven".

Glassmaking is a collaborative process. Two of Layton's colleagues – Bruce Marks and Louis Thompson – are busy making a bowl. They begin by melting some solid coloured glass in the "glory hole" – the reheating chamber. When the glass starts to droop they pull it out and begin work. "Everything is built up in layers – by layering you get amazing colour variations," says Layton.

A lot of glassblowing is about timing: how long you heat it, the time you have to shape it before it hardens (about 30 seconds) and when to stop before you potentially destroy it. Along with a very steady hand and an ability to withstand temperatures of up to 45C in the open, studio glassblowers need to remain calm, methodical and above all keep working without losing concentration for a second.

"In a factory, if you spend more than three minutes on a piece, that would be a long time," says Layton. "We might spend an hour and a half on a piece."

Thompson sits on the "blowing bench" with two parallel bars projecting either side, upon which he rests his blowing rod (a hollow iron rod) with gathered molten glass at its end.

One hand constantly turns the rod, while the other shapes the glass using wads of wetted paper. The process is spellbinding. It looks effortless; but the reality is, it's fiendishly hard and anything can go wrong at any moment.

Thompson is about to blow a bubble into the glass. Because the glass is so hot, the air inside expands and creates a bubble. Further layers of clear or coloured molten glass are gathered over the previous ones, and Thompson blows to create a large, hollow sphere that will eventually become a bowl.

The pair move swiftly on to the next bowl. Meanwhile, Layton takes me to see the architectural wing of the workshop where I view pictures of privately commissioned glass panels and a large geometric hanging sculpture now installed inside John Lewis, Liverpool.

Most recently though, they've been back doing what glassblowers know best: making goblets – 300 of them in fact – in traditional medieval style, for a forthcoming Robin Hood film. It's as if nothing has changed, and they're back in an age where all functional objects took time to handcraft, and 99p pile-em-high-sell-em-cheap stores were an untold craze of the future.